People usually extract effective substances from solid materials to prepare liquid, such as boiling medicinal materials and cooking soup. A conventional method for preparing liquid mainly uses a “boiling” method; i.e., put raw materials into water, heat the water so that the effective substances in the materials are released under the function of high temperature and dissolved into the water that becomes a liquid containing effective substances of the materials. This method is relatively quite complicated. In order to fully extract the effective substances in the materials, people usually carry out a control, in view of actual situations, of quantity of added water, boiling time, quantity of remained water and fire power. These operations are difficult to carry out successfully without rich experience. For each type of materials, these operations need to be adjusted according to differences of environment and actual situations. Even though an experienced person keeps monitoring these operations, the prepared liquid may become concentrated and the materials burnt to be unusable due to his accidentally careless behavior. Thus, automatic control for these operations is difficult to achieve. Meanwhile, the originally added water may gradually evaporate in the process of boiling and the non-volatile harmful substances will be accumulated in the prepared liquid which is harmful to human bodies if the liquid is used for drinking. In addition, the volatile effective substances may be volatilized into the air in the process of boiling, which causes amount decrease and waste of effective substances.